Non-volatile flash memory (such as NAND flash memory) is a common memory chip. Data stored in the NAND flash memory will not be lost if the power is turned off and is electrically erasable and writable in a system. The NAND flash memory has the advantages of a large capacity and a fast rewriting speed and is suitable for storage of a large amount of data. The NAND flash memory has become the mainstream of the market due to a high integration level and a low cost.
FIG. 1 is a simplified schematic diagram of a common NAND flash memory including a memory cell array 11, a word line selection unit 12, a bit line selection unit 13, a charge pump 14 and a control unit 15. The memory cell array 11 is an array of memory cells which are arranged along the word lines and bit lines. Specifically, the memory cell array 11 is composed of multiple blocks, each block is composed of multiple pages, and each page is composed of memory cells connected by word lines and bit lines. Operations on memory cells generally include: an erase operation, a program (write) operation and a read operation. The erase operation is performed in blocks. The program operation and the read operation are performed in pages. For example, when an erase operation is performed on memory cells, the control unit 15 selects a specific erasing block through the word line selection unit 12 and the bit line selection unit 13, and controls the charge pump 14 to apply an erasing voltage to corresponding memory cells.
The erase operation can be easily performed on a fresh NAND flash memory chip that has just been put into use. Generally, after just 3 to 5 erase operations, data can be erased successfully. For a cycled NAND flash memory chip that has been put into use for a long time, with the increasing number of erase operations on the NAND flash memory, the erase operation becomes more difficult. Thus, how to make a cycled flash memory chip that has been put into use for a long time be erased successfully has been what those skilled in the art are trying to find out.